A storage system is demanded of meeting the needs of high-speed access, large capacity and low cost. Thus, a storage system includes a storage device of a hierarchy which has a high bit unit price but enables high-speed access, and a storage device of a hierarchy which yields low-speed access but has a low bit unit price, and performs automated hierarchy control of storing data of high access frequency in the storage device of a hierarchy which enables high-speed access, and storing data of low access frequency in the storage device of a hierarchy which is low speed but has a low bit unit price and large capacity.
With the automated hierarchy control, access frequency to data of relocation units of high-speed hierarchy and low-speed hierarchy is monitored, data of relocation units of high access frequency in the low-speed hierarchy is relocated to the high-speed hierarchy, and data of relocation units of low access frequency in the high-speed hierarchy is relocated to the low-speed hierarchy.
Automated hierarchy control is described, for example, in Japanese Patent Application Laid-open No. 2011-227898, Japanese Patent Application Laid-open No. H8-77073, and Japanese Patent Application Laid-open No. H6-309224.
Nevertheless, with the automated hierarchy control described above, when access to the storage device of a low-speed hierarchy increases suddenly and reaches a performance limit, a delay in response, in which the response time from access to response becomes longer, will arise. Consequently, the access frequency to the relocation units in the low-speed hierarchy will become lower than the access frequency that was actually requested. Thus, there are cases where data, which is to be relocated to a to higher speed hierarchy, continues to remain in a low-speed hierarchy, and the overall response time is not optimized. In other words, even though the overall storage system may have some leeway in terms of performance, when the storage system reaches a performance limit by becoming locally an overload state, it is not possible to properly evaluate the originally requested access frequency, and the intended relocation control cannot be performed.